What is the key difference between SECNAVINST and DODD directives?

The key difference highlighted by the correct choice is that SECNAVINST specifically pertains to directives issued by the Secretary of the Navy, which apply exclusively to the operations and policies of the Navy and the Marine Corps. This distinction is important because it emphasizes the unique governance structure of the U.S. military, where different branches operate under their own set of guidelines and instructions tailored to their specific missions and organizational needs.

In contrast, DODD refers to Department of Defense Directives, which are broader in scope and apply across all branches of the U.S. military. This framework helps to standardize policies and procedures that affect the entire Department of Defense rather than being limited to a specific branch like the Navy or Marine Corps. Understanding this difference is crucial for those in military operations and administration, as it impacts the implementation of policies and compliance within each service branch.